"Kate was saying 'hi,' 'hello' to everyone she saw and was smiling ear-to-ear despite the bad weather," an observer told Us Weekly of Middleton before she boarded. "William walked a step or two in front of her the whole time. She looked gorgeous in red -- really slim and healthy looking."

Middleton, 30, wore a striking red Alexander McQueen dress, Sylvia Fletcher-designed hat and carried a red and gold clutch. When it got chilly, she wrapped up in a Strathearn tartan scarf. William wore his Flight Lieutenant No. 1 dress uniform and his brother, Prince Harry, wore the No. 1 ceremonial dress of the Blues and Royals.

Prince Charles and Camilla, Duchess of Cornwall joined them on the boat -- which was decorated by 10,000 flowers -- and a trumpet fanfare announced their departure after Queen Elizabeth II and Prince Phillip boarded last.

Middleton's family -- sister Pippa, brother James, father Michael and mother Carole -- followed on the Elizabethan steamer boat, giving big smiles to fans. The 1,000 boats in the flotilla officially broke the Guinness World Records title for the world's largest parade of boats.